Rare earth magnets (especially permanent magnets) typically exemplified by Nd—Fe—B based magnets exhibit very high magnetic properties. Since use of the rare earth magnets can realize downsizing, output power enhancement, density enhancement, environmental burden reduction and the like of electromagnetic devices and electric motors, application of the rare earth magnets is being investigated in a wide range of fields.
However, in order to achieve practical application, it is requested that good magnetic properties of the rare earth magnets are exhibited stably for a long time even under severe environments. Therefore, research and development are actively conducted to improve coercivity, which is effective in providing heat resistance (demagnetization resistance), while maintaining or improving high residual magnetic flux density of the rare earth magnets. One of the most effective methods is to diffuse a diffusing element such as dysprosium (Dy) and terbium (Tb), which is a rare earth element having high anisotropic magnetic field (Ha), into grain boundaries of main phase crystal (e.g., Nd2Fe14B-type crystal) and the like. This diffusion treatment allows an improvement in crystal magnetic anisotropy and suppression of generation of starting points of reverse magnetic domains while suppressing replacement of Dy or the like in crystal grains, and accordingly allows an improvement in coercivity while suppressing a decrease in residual magnetic flux density.
By the way, this diffusion treatment can be performed in a variety of methods. One example of the methods is a powder mixing method in which magnet powder comprising a raw material alloy of a rare earth magnet (hereinafter referred to as a “rare earth magnet alloy”) is mixed with diffusing powder containing a diffusing element and a compact of the obtained mixed powder is sintered or the like to perform the aforementioned diffusion treatment. Another example of the methods is a coating method in which diffusing powder or the like is coated on a surface of a magnet material to be subjected to diffusion treatment and then heat treatment is applied to the coated material to perform the diffusion treatment. Furthermore, in order to perform efficient diffusion treatment while suppressing the amount of Dy, or the like, which is a scarce element, to be used, a vapor method has recently been proposed in which a diffusing element is efficiently diffused into an inside of a magnet material by exposing the magnet material to vapor of the diffusing element. Description of this vapor method is found, for example, in the following patent documents.